Conventionally, nitride semiconductor has been utilized or studied for a light emitting device and a high-power semiconductor device. In a nitride semiconductor light emitting device, InGaN is often used for a quantum well layer included in its light emitting layer, and light emitting devices of a wide range of colors from blue to orange can be formed by adjusting the In content in InGaN. In recent years, blue and green light emitting diodes, violet semiconductor laser and others have been developed utilizing the characteristics of the nitride semiconductor light emitting devices.
In the case of forming a light emitting device using a quantum well layer of nitride semiconductor containing In, however, there are problems as follows. The nitride semiconductor containing In is very unstable in its chemical thermal equilibrium state. Thus, it is liable to experience phase separation (concentration separation) into regions of high In contents and regions of low In contents, which causes mottling in the light emitted from the light emitting device. In addition, the regions having the high In contents as a result of the concentration separation is liable to emit no light, leading to degradation of luminous efficiency.
On the other hand, Japanese Patent Laying-Open No. 10-270804 discloses a nitride semiconductor light emitting device having a light emitting layer including GaNAs (or GaNP or GaNSb) well layers and GaN barrier layers.
In this nitride semiconductor light emitting device, however, it is difficult to obtain steep composition change at the interface between the GaNAs well layer and the GaN barrier layer. Such insufficient steepness at the interface between the well and barrier layers causes increase of half-width of emission peak, increase of mottling, and degradation of luminous intensity (or gain reduction). Further, the fact that it is difficult to obtain steepness at the interface between the well and barrier layers means that it is difficult to form the multiple quantum well structure with these layers. Still further, the problems in the GaNAs well layer/GaN barrier layer may similarly arise also in GaNP well layer/GaN barrier layer and in GaNSb well layer/GaN barrier layer.
In view of the above-described problems of the prior art, an object of the present invention is, in a nitride semiconductor light emitting device including a quantum well layer of nitride semiconductor containing In, to suppress phase separation in the well layer to prevent mottling in light emitted from the device and improve luminous efficiency thereof.
Another object of the present invention is to improve steepness at an interface between well and barrier layers to provide a nitride semiconductor light emitting device of high luminous efficiency.